1. Technical Field
The present invention relates to synchronization apparatus for synchronizing an oscillator output to an input synchronization signal over a wide range of frequencies.
2. Description of the Related Art
Conventional synchronization apparatus typically includes a phase-locked loop for locking the output frequency of an oscillator to that of an independently produced input signal. For example, a raster-scanned cathode ray tube (CRT) display device such as a computer visual display unit or a television receiver includes a phase-locked loop for locking an output line scan signal for scanning lines of video onto the CRT screen to an input line synchronization (sync) signal produced by the video source.
In general, a phase-locked loop of this kind is associated with a "capture range" of frequencies sometimes referred to as a "pull-in range". The capture range specifies the range of frequencies centered on the frequency of the line sync signal within which the line scan signal can be brought into synchronization with the line sync signal. If the frequency of the line scan signal is outside the capture range of the phase-locked loop when the line sync signal is first supplied to the display device, the phase-locked loop cannot synchronize the line scan signal to the line sync signal.
The phase-locked loop is also associated with a "hold range" of frequencies, which is usually slightly wider than the capture range. The hold range specifies the range of frequencies centered on the line sync frequency within which the phase-locked loop can hold the line scan signal locked to the line sync signal following, for example, a step change in the line sync frequency. If the step change in the line sync frequency leaves the line scan signal outside the hold range, the phase-locked loop cannot hold the line scan signal in synchronization with the line sync signal. Thereafter, the phase-locked loop can only recover synchronization if the line scan frequency is brought within the capture range.
Many computer display systems can generate video signals in a number of picture display formats. Each display format is characterized by a different combination of raster line and frame sync frequencies. The variation in line sync frequency between different display formats can be relatively wide, from 32 kHz to 64 kHz for example. This variation is much wider than the capture and hold ranges of the phase-locked loops in the synchronization apparatus of conventional display devices, which are typically no greater than 6 kHz. Therefore, a change in display format in the computer display system may produce a step change in line sync frequency that is too large for the phase-locked loop of conventional synchronization apparatus in a display device to follow, leading to a loss of picture synchronization on the screen of the display device.